1. Field of Invention
The present disclosure relates to power electronic technology. More particularly, the present disclosure relates to a switch unit including a contact switch.
2. Description of Related Art
With the ever-increasing seriousness of energy-related problems in recent times, the development of new ways in which to generate energy (e.g., wind energy, solar energy, etc.) is attracting the attention of those involved in research and development in various areas. In a power generation system, the amplitude and frequency of the output AC (alternating current) of the power generator is not consistent with those of the AC power grid. Therefore, the output AC power of the power generator is converted into a DC (direct current) power by means of a converter, and subsequently the DC power is inverted into another AC power which is consistent with the AC power grid in frequency and phase, so as to realize grid-connected power generation. In the process of converting the AC power into the DC power and subsequently inverting the DC power into another AC power, electronic components (e.g., power switching valves) are controlled by PWM (pulse width modulation), and the AC-DC conversion and DC-AC conversion are realized by turning on or off these power switching valves.
In a wind power generation system, for example, a traditional wind power converter is directly connected with a wind turbine or a controllable contact switch is connected in series between the wind power converter and the wind turbine. When the system fails, if the wind power converter is directly connected with the wind turbine, the rate of rotation of the wind turbine is limited in a range only by adding external loadings. For example, when the wind turbine rotates at a high speed, the voltage at the input interface of the converter is also very high. Thus, in order to protect the power components in the converter from being damaged when subjected to high voltage, typically, a set of dump loads is additionally arranged at the input interface of the wind turbine and the converter, or on the post-grade bus of a rectifier bridge, so that the voltage at the interface is reduced to an operation range which is allowed by the power component. However, since the wind turbine is directly and continuously connected with the converter, and the power of the dump loads is limited, the voltage limiting capacity of the dump loads is also limited. Furthermore, the switching response rate of the dump loads is slow, so that once the system fails unexpectedly, the reliability thereof cannot be ensured.
Furthermore, the contact switch includes a control terminal and a main contact, and the control terminal may be a coil. When an active signal (for example, power on of the coil) is provided to the control terminal, the main contact starts to operate so as to turn on the contact switch. When an invalid signal (for example, power down of the coil) is provided to the control terminal, the main contact starts to operate so as to turn off the contact switch. When the controllable series connected contact switch is used, a special arc extinction device is needed, so as to reliably cut off the electric connection between the converter and the wind turbine when a fault happens. However, such a contact switch is mostly applied to a high power-grade wind power generation system, and the cost of a contact switch having an arc extinction function is high. Additionally, if a controllable contact switch is directly used as the input switch (having no arc extinction device), since the wind turbine winding itself has a large equivalent inductance (for example, a 20 kW power generator has an equivalent inductance of about 18 mH to 40 mH), at the point in time that the switch is turned off, the energy stored by the equivalent inductance cannot be released in time, so that the generated arc voltage is loaded on two ends of the contact switch and thus the contact switch cannot operate normally or the usage life of the contact switch is reduced.
In view of this, many in the industry are endeavoring to find ways in which to design a novel switch unit, so as to quickly cut off the electric connection between the wind turbine and the converter when the system fails and thereby protect the contact switch.